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Small grid-tie inverters? - Page 14

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Posted by Don Kelly on June 24, 2008, 3:19 am


Daestrom is correct- a 250W inverter will do nothing to the grid- it won't
even notice it. However the inverter will notice it- and there is definite
chance of letting the magic smoke out of the inverter (or, preferrably,
tripping the breaker).  Bob G might  find that turning a "stand alone"
inverter into garbage is expensive although the utility won't notice such a
small unit.

   If a GW system is so close to the stability point that a 250W inverter
out of phase can bring it down, then don't switch on (or off) the tea kettle
until you have your candles on hand.  (typically the system is not operated
anywhere near either the steady state or the transient stability limit by at
least a factor of 2 ).

Proper "grid tie" inverters are automatically synchronized to the grid
(protecting the inverter -not the grid).   In some the local oscillator is
the grid and  in others, the grid voltage is sensed and the phase of the
local oscillator adjusted automatically to match.

 Safety concerns are there but they are not related to
synchronization/blackouts.   Most  are related to energization of lines that
are expected to be "dead" and proper practice is to ground these lines (on
both sides of the work area) before working on them- not just assuming that
there is no back feed. That might just take care of (overload) an illicit
small converter.

While  I suspect that a 250W inverter plugged into an outlet will not be
able to handle the magnetizing current of the local utility transformer, the
regulations in Canada and the US, as in OZ err on the safe side as they
should.  This means that there should be no connection to the grid that is
not approved- at any level- which is what you have said.


Don Kelly dhky@shawcross.ca
remove the X to answer

Posted by Daniel Who Wants to Know on June 25, 2008, 3:43 am

The simple non electronic (albeit less efficient) way to feed power back
would be to have a good DC motor coupled to a standard induction motor.
Most people don't realize that induction motors instantly turn into
generators if you use an external source to attempt to turn the shaft faster
than the synchronous speed and assuming you don't have any capacitors
connected in parallel will simply stop generating and allow the shaft to
spin free when the grid power goes down.

Posted by Vaughn Simon on June 25, 2008, 10:05 am

   That is exactly how it was done on nuclear submarines back in my Navy days.
But that is old technology and we have better ways of doing it now.


Posted by Solar Flare on June 25, 2008, 11:58 am
 Dynamic braking is still a good engineering design.

Posted by daestrom on June 25, 2008, 8:36 pm
 Vaughn Simon wrote:

Ugh... no it wasn't.  The MG sets on nuc submarines were *synchronous*
motors on the AC side, not *induction* motors.  These could reverse power
flow just as easily but provided much better voltage and frequency
regulation when running isolated from the turbine-generator to power the
vital busses.


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